1. Field of the Invention
The invention relates to a rolling bearing unit.
2. Description of the Related Art
Conventionally, grease lubrication has been widely adopted to lubricate rolling bearing units. In the case of grease lubrication, in order to maintain satisfactory lubrication performance even under severe lubrication conditions such as a high-temperature condition, a high-speed rotation condition and a high-load condition, a grease reservoir is formed in an outer ring spacer for a bearing and lubricant reserved in the grease reservoir is supplied to the bearing. Bearing units having such a mechanism are described in WO 2010/010897 and Japanese Patent Application Publication No. 2005-180629 (JP 2005-180629 A).
WO 2010/010897 describes a rolling bearing unit including an inner ring, an outer ring, a plurality of balls, a cage, a seal and an annular grease reserving member. The balls are rolling elements interposed between the inner ring and the outer ring. The cage retains the balls located at prescribed intervals in the circumferential direction. One axial end of an annular space defined between the inner ring and the outer ring is sealed by the seal. The grease reserving member is disposed near an annular groove located adjacent to a guide surface of the outer ring, which guides rotation of the cage. The lubricant is reserved in the grease reserving member.
JP 2005-180629 A describes a rolling bearing including an inner ring, an outer ring, a plurality of rolling elements, a grease reservoir forming member and a clearance forming piece. The rolling elements are interposed between raceway surfaces of the inner and outer rings. The grease reservoir forming member is disposed in contact with the outer ring. The clearance forming piece extends from the grease reservoir to a position near the raceway surface of the outer ring to define a clearance along an inner peripheral face of the outer ring.
If such a rolling bearing unit in which grease is sealingly enclosed is used for a long time, cracks (voids) may be formed in the grease in the grease reservoir as base oil that is contained in the grease is supplied. If such cracks are successively connected to each other over the entire circumference of the bearing, at a position near a communication passage between the grease reservoir and the bearing, the base oil in the grease near the bearing is depleted although the grease remains in the grease reservoir. As a result, a supply of the base oil is interrupted. In the bearing unit in which a supply of the base oil is interrupted, the bearing lubrication performance is lost after the base oil in the bearing is completely consumed. After the bearing lubrication performance is lost, it is difficult to keep using the bearing unit for a long time.
JP 2005-180629 A describes a configuration in which a small clearance is defined between the outer ring and the clearance forming piece. With this configuration, the base oil that is contained in the grease is supplied to a position near the raceway surface of the outer ring, in order to prolong the service life of the bearing having a satisfactory lubrication performance. However, with the technique described in the JP 2005-180629 A, the shape of the clearance forming piece needs to be accurately designed to form a small clearance. This complicates the configuration of the rolling bearing unit.